4G networks are the fourth generation of mobile telecommunication technology standards, and include the WiMAX and LTE-Advanced network standards. The 4G networks include a new architecture to support a small scale evolved Node B (eNodeB), which may be installed in private homes (known as a femto access point, or femtocell), or outdoor areas (known as metrocells or picocells, depending on the coverage area). These cells are collectively known as small cells.
Small cells can reside in a coverage area of a larger macro cell, and must each include a Physical Cell ID, PCI, to identify itself to other cells and User Equipment, UE, on the mobile network. In 4G networks, cells can self-allocate their PCI by selecting from a pool of 504 possible PCIs, the selection process depending on the addresses used by neighbouring cells (which differs from the centralized PCI allocation procedure of previous mobile telecommunications networks). For example, an eNodeB for a new cell initially selects a random PCI from the possible PCIs, and determines the PCIs in use by neighbouring cells after receiving a measurement report from a User Equipment, UE, or by sniffing neighbouring cells. The eNodeB may then alter its PCI such that there are no collisions or confusion with any neighbouring cells. The skilled person will understand that a PCI collision occurs when two neighbouring cells have an identical PCI, and PCI confusion occurs when a UE can detect two cells with identical PCIs. A violation of either of these rules is known as a PCI conflict.
The PCI value is very important during the cell selection, handover and reselection processes. When a UE is switched on and does not have an up to date list of neighbouring cells, the UE will scan for eNodeBs belonging to its network using the Public Land Mobile Networks stored in its Universal Subscriber Identity Module. The UE will decode primary and secondary synchronization signals from eNodeBs in the Public Land Mobile Network (and calculate the PCIs for the eNodeBs from these synchronization signals), and measure their signal strength. The UE will then attempt to register with an eNodeB for its highest priority Public Land Mobile Network.
As the UE moves between cells, a cell handover process is used to transfer the UE from a serving cell to a destination cell. In dense cell deployment areas (which is common for small cells), the handover is problematic if there is a PCI conflict. For example, if the serving cell has two neighbouring cells with identical PCIs (i.e. such that there is PCI confusion), then the handover may not take place. To overcome this, an additional identifier for the destination cell (a Global Cell Identifier) is sent to the serving cell, which enables differentiation of the two neighbouring cells with identical PCIs.
When a UE is camped on a cell, it may enter an Idle mode (if, for example, there is no traffic flowing to it) and periodically initialize to check the signal strength of the eNodeB. If the signal strength drops below a threshold value, the UE will perform a cell reselection process to select the most appropriate cell (e.g. a cell having a greater signal strength). The cell reselection process can be aided by a neighbour list that is broadcast by the serving cell. The neighbour list can contain the PCIs for up to 16 neighbouring cells, along with a Q offset value for each PCI. The Q offset is an offset between −24 dB and +24 dB, which the UE adds to a measured signal strength for that cell (along with configured hysteresis values). The UE may then determine which cell to select by: identifying a plurality of neighbouring cells (using the PSS and SSS signals), measuring the signal strength from each cell, add the Q offset to the measured signal strength for each cell of the plurality of neighbouring cells that is also present on the neighbour list, and select a cell to connect to based on the signal strength including the Q offset. The Q offset value is therefore used to make neighbouring cells more or less attractive, for example, if a cell has a high level of traffic and a neighbouring cell has a low level of traffic, it may give the neighbouring cell a positive Q offset in the neighbour list to encourage the UE to connect to that cell instead.
The present inventors have identified a problem with the cell reselection process when there is a PCI conflict. That is, the neighbour list broadcast by each eNodeB only includes the PCI value to identify a cell, such that it cannot differentiate between two cells having the same PCI. In the event the neighbour list includes a PCI conflict, the neighbor list will not be able to list the two different Q offsets for the same PCI.
This problem is particularly relevant for small cells, as it is likely that many small cells will be deployed in a small geographical area (for example, a block of flats in which each flat has its own small cell). In such circumstances, there may be more than 504 small cells deployed within a macrocell coverage area, giving a high probability that PCI duplication occurs. Furthermore, as the neighbour list may only include a maximum of 16 cells, there is also a high probability that PCI duplication occurs on this list.
International Patent Application Number 2013/078573 A1 discloses a method and apparatus for PCI allocation in a cellular network, wherein the PCI for a second cell is allocated based on a distance relation between a first cell and the second cell and radii of first and second boundary circles of the first and second cells respectively.
US Patent Application Publication Number 2011/0086652 discloses a method and apparatus for allocating PCIs in a wireless communication system by using a PCI reuse factor and the received signal strength of each of the cells using a set of candidate PCIs.
It is therefore desirable to alleviate some or all of the above problems.